This spell does not seem to be working properly and apparently has been a known issue for some time. The temporary hit points gained from this spell disappear immediately after you are hit by an enemy - regardless of how much damage they do.
i.e. if I get 20 temporary HP and an enemy hits me for 5 damage, all 20 temporary HP are gone. I have noticed this even in situations where the temporary HP shouldn’t disappear at all. The Boggard casters that use Flame Lash will knock out all the temporary HP even if I have Resist Fire up (which otherwise completely counters the damage the spell does).